Banded iron formation

Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock that are almost always of Precambrian age.

A typical BIF consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shales and cherts, often red in color, of similar thickness, and containing microbands (sub-millimeter) of iron oxides.[1]

An ashtray carved out of a soft form of banded ironstone from the 3.5 billion year old Barbeton Supergroup in South Africa. The red layers were laid down during the daylight hours when Archaean photosynthesizing cyanobacteria produced oxygen that immediately reacted with dissolved iron compounds in the water, to form insoluble iron oxide (rust). The white layers are sediments that settled during the night when there was no oxygen in the water.[7]

The conventional concept is that the banded iron layers were formed in sea water as the result of oxygen released by photosyntheticcyanobacteria. The oxygen then combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming a thin layer on the ocean floor, which may have been anoxic mud (forming shale and chert). Each band is similar to a varve, to the extent that the banding is assumed to result from cyclic variations in available oxygen.

It is assumed that initially the Earth started with vast amounts of iron and nickel dissolved in the world's acidic seas. As photosynthetic organisms generated oxygen, the available iron in the Earth's oceans precipitated out as iron oxides.

At a suspected tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced periods of free oxygen in the surface waters, alternating with periods of iron oxide deposition.

Until 1992,[9] it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally, and iron-rich waters could form and then come into contact with oxygenated water.

An alternative explanation of these later deposits was undergoing much discussion as part of the Snowball Earth hypothesis. Several hypotheses exist for the initiation of the Snowball Earths. The initiation mechanisms which include the breakup of the early equatorial supercontinent (Rodinia), the first colonization of the land by early lichens and fungi and variations in the Earth's axial tilt are yet to be convincingly identified. In a Snowball Earth state the earth's continents, and possibly seas at low latitudes, were totally covered in an ice age.

If this was the case, Earth's free oxygen may have been nearly or totally depleted during a severe ice age circa 750 to 580 million years ago (mya). Dissolved iron then accumulated in the oxygen-poor oceans (possibly from seafloor hydrothermal vents). Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron.

Another mechanism for BIF's, also proposed in the context of the Snowball Earth discussion, is by deposition from metal-rich brines in the vicinity of hydrothermally active rift zones.[10] Alternatively, some geochemists suggest that BIFs could form by direct oxidation of iron by microbial anoxygenic phototrophs.[11]

Northern Minnesota's banded iron formations lie directly underneath a thick layer of material only recently recognized as ejecta from the Sudbury Basin impact. At the time of formation the earth had a single supercontinent with substantial continental shelves.

An asteroid (estimated at 10 km across) slammed into waters about 1,000 m deep some 1.85 billion years ago. Computer models suggest that the tsunami would have been at least 1,000 m at the epicentre, and 100 m high about 3,000 km away. Those immense waves and large underwater landslides triggered by the impact stirred the ocean, bringing oxygenated waters from the surface down to the ocean floor.[4]

Sediments deposited on the seafloor before the impact, including BIFs contained little if any oxidized iron (Fe(III)), but were high in reduced iron (Fe(II)). This Fe(III) to Fe(II) ratio suggests that most parts of the ocean were relatively devoid of oxygen.

Marine sediments deposited after the impact included substantial amounts of Fe(III) but very little Fe(II). This suggests that sizeable amounts of dissolved oxygen were available to form sediments rich in Fe(III). Following the impact dissolved iron was mixed into the deepest parts of the ocean. This would have choked off most of the supply of Fe(II) to shallower waters where BIFs typically accumulated.

The geological record suggests that environmental changes were happening in oceans worldwide even before the Sudbury impact. The role the Sudbury Basin impact played in temporarily shutting down BIF accumulation is not fully understood.